LEDs have been massively produced at present, and most of the LEDs are used for lighting and display. A plurality of LEDs can be connected in series to form one or more LED strings, and an LED driver drives the LED string to emit light. A conventional LED driver has varied structures, one of which is shown in FIG. 1. An LED driver 10 is coupled to an LED string 50, and drives the LED string 50 according to image brightness information Sbr. The LED driver 10 may control an LED current IL flowing through the LED string 50 according to different image brightness information Sbr (that is, different image brightness information Sbr corresponds to a different LED current IL), so as to control the brightness of the LED string 50. Because the brightness is controlled within a relatively wide operating current range, the image brightness information Sbr needs to be programmed also within a relatively wide range.
As shown in FIG. 1, the LED driver 10 includes an LED controller 11, a current source 13, a first current mirror 15, a second current mirror 17, and a drive transistor 19. The LED controller 11 receives the image brightness information Sbr, generates a digital code signal Code according to the image brightness information Sbr, and transmits the digital code signal Code to the current source 13, to adjust a reference current Iref flowing through the current source 13. Further, if the digital code signal Code is 8-bit data, the LED controller 11 converts the image brightness information Sbr into an 8-bit digital code signal Code, to adjust the reference current Iref flowing through the current source 13.
The first current mirror 15 generates a first current I1 according to the reference current Iref and a first rate K1 of the first current mirror 15. Subsequently, the second current mirror 17 generates the LED current IL flowing through the LED string 50 according to the first current I1 and a second rate K2 of the second current mirror 17. In addition, the LED controller 11 generates a pulse-width modulation (PWM) signal according to the image brightness information Sbr, to turn on/off the drive transistor 19. In this way, the LED string 50 is driven, and the brightness of the LED string 50 is controlled according to the LED current IL. It should be noted that, the LED current IL is equal to a product obtained by multiplying the reference current Iref, the first rate K1, and the second rate K2 together, where the first rate K1 multiplied by the second rate K2 is a constant.
Therefore, the first rate K1 and the second rate K2 are nonadjustable constants conventionally, and the brightness (corresponding to the LED current IL) is controlled within a relatively wide operating current range (for example, from a small current 20 mA to a large current 200 mA). Therefore, the image brightness information Sbr needs to be programmed also within a relatively wide range. However, if running within a relatively wide operating current range, the conventional LED driver 10 is unable to correctly maintain the LED current IL in a preset variation range.
FIG. 2 simulates a variation of the LED current IL in the case where the conventional LED driver 10 runs within a relatively wide operating current range (that is, from a small current 20 mA to a large current 200 mA). As shown in FIG. 2, the curves CV1 and CV2 separately show a result obtained through a Monte Carlo method performed for different numbers of times. In the operating current range from the small current 20 mA to the large current 200 mA, the variation of the LED current IL gradually decreases. Therefore, if the preset variation range is set from −2% to +2%, the variations shown by the curves CV1 and CV2 cannot be maintained in the preset variation range all the time. A conventional solution is to correct the variation of the LED current IL in sections according to the simulation diagram of FIG. 2. As shown in FIG. 2, an operator divides the whole operating current range (that is, from 20 mA to 200 mA) into four sections A, B, C, and D according to the result shown by the actual simulation diagram, and then adjusts the variation in each section of A to D, such that the adjusted variation is maintained in the preset variation range. However, the conventional solution increases the test time and cost, and the operator cannot correctly decide an adjustment amount for each section, causing an unsatisfactory effect after the adjustment.